This invention relates in general to the field of synchronous switching operations in power lines. In particular, the present invention relates to a method and an apparatus for optimizing synchronous switching operations executed by a switching device operatively coupled to an AC power line.
As it is well known, power systems for transmitting and distributing electricity from power sources to various loads and users are equipped with several types of protecting switchgear, such as high-voltage circuit breakers. Such switchgear are typically adapted for intervening under determined operating conditions so as to ensure a proper functioning of an associated power line and of loads/users connected therewith.
Voltage and current transients generated during opening/closing switchgear operations are of increasing concern for the electric utility industry. These concerns include both power quality issues for voltage-sensitive customer loads, and excessive stresses on power system equipment. Some proposed solutions for reducing switching transients include circuit breaker pre-insertion devices, such as resistors or inductors, and fixed devices such as arresters and current limiting reactors.
A solution finding increasing popularity is the so-called synchronous switching method, sometimes also referred to as the point-on-wave switching. Synchronous switching is performed by dedicated electronic devices, indicated in the art as synchronous switching devices, which control the operations of the associated switchgear. Upon receiving a close or a trip command, a synchronous switching device delays the energization of the circuit breaker control coils by a few milliseconds. In this way, the current inception in the case of a close command, or the contact separation in the case of an opening or trip command, is expected to coincide with or be in a narrow window around a certain point on the AC wave which is known to reduce switching transients. For synchronous closing, this point is often the voltage zero crossing. Applications where it is beneficial to close the contacts on or near the voltage zero crossing include the energizing of capacitor banks and energizing of unloaded lines or cables. For opening synchronous switching operations, the targeted point-on-wave is a point on the current wave chosen to minimize the probability of unwanted restrikes or reignitions, for instance two milliseconds after a zero crossing. Synchronous opening can be employed for shunt reactors de-energizing as an example.
Two conditions must be met to successfully employ the concept of synchronous switching. First, the targets or points-on-wave that will result in the desired reduction of switching transients must be identified. Second, a method must be used to ensure that current inception (or contact separation) for each phase occurs as close as possible to the selected targets. This latter involves algorithms for the prediction of the future course of waveforms, as well as algorithms for the calculation of exact switchgear operating times for each pole based on past operations and/or external parameters, such as temperature, control voltage, hydraulic pressure etc.
The identification of the targets or points-on-wave that will result in the desired reduction of the switching transients is traditionally performed a-priori using system studies or general knowledge of the equipment being energized (or de-energized), such as capacitor banks, transformers, transmission lines, shunt reactors etc. These targets are programmed into the synchronous switching device and usually remain unchanged as long as the synchronous switching device is used for the same application. Some implementations of synchronous switching adapt the target for synchronous closing depending on dielectric properties, in particular the rate of decrease of dielectric strength (RDDS), of the circuit breaker.
However, in spite of theoretical knowledge and system studies performed to identify the targets on the voltage (or current) AC waveform for each pole to close (or open) that will result in the desired reduction of switching transients, some switching transients may still occur due to deviations of the theoretical behavior of the system from its practical behavior.
Hence, there is a potential to further reduce the severity of switching transients or even completely eliminate them and it would be desirable to provide a solution which allows to further optimize the selection of the targets on the AC waveform. This solution is provided by the method and apparatus of the present invention.